US intelligence chiefs refused to pass to Britain full details of a “Mumbai-style” terrorist plot targeting London because of concerns that secret sources could be exposed in our courts.

The CIA had told MI6 18 months ago of al Qaeda plans against several cities in Europe, which suggested that the seizing and killing of hostages could be involved.

The plot was in its early stages, but the US agents stopped short of passing on everything they knew in order to protect the identity of their sources.

British intelligence officers were forced to look deeper into the information they had received, with their investigations ultimately leading to the identification of several terror suspects.

Justice Secretary Ken Clarke, defending plans to extend secret court hearings to civil cases, said that the case highlighted the reluctance of US intelligence agencies to fully share information after the Binyam Mohamed case in 2010.

A court ruling led to the publication of seven paragraphs of intelligence material supplied in confidence to Britain by the CIA relating to the torture of the former Guantanamo Bay detainee.

Ministers claimed at the time that the disclosure would harm the flow of future information from the US.

Mr Clarke said: “The Americans have got nervous that we are going to start revealing some of their information, and they have started cutting back on what they disclose.”

A senior British security source added: “The urgent threat-to-life operational material is still coming. But we see strong signs of a greater reluctance to share some of the other stuff — the building blocks, the bits that let you put the jigsaw together.”

David Anderson QC, the Government’s terrorism watchdog, today said that there had been a “diminution” of intelligence sharing, but insisted the Government’s “well-founded fears” about a drop in intelligence sharing should not be used as a “scare tactic” to introduce secret trials in “unrelated” civil cases.